The purpose of this project is to test the hypothesis that hepatitis C virus (HCV) infection persists due to arrested development of CDS T cell-mediated antiviral responses during the ransition from acute to chronic phase by CD4 dependent mechanisms. During the previous funding period we have developed an infrastructure to obtain large numbers of peripheral blood mononuclear cells (PBMC) from acutely-infected individuals, and used overlapping peptides encompassing the entire HCV polyprotein to characterize IFNgamma-mediated immune responses. Most subjects responded to multiple epitopes, with peak recognition around 6 months then decreasing during the following 12-18 months. New specificities were not detected after the first 6 months, despite ongoing amino acid replacements. Amino acid replacements in T cell epitopes frequently represent escape mutants, and most epitopes develop amino acid replacements. We therefore propose to investigate the mechanisms of cellular immune failure during the transition between acute and chronic infection. Aim 1: To define molecular markers of CD8+ T lymphocyte failure in PBMC from persons with acute HCV infection. We will determine the timing and breadth of response, measure cell-surface and intracellular markers of CDS phenotype and maturation. We will correlate these findings with responses in lymphocytes obtained from livers in a well-characterized subset of volunteers to assess the degree of compartmentalization. Aim 2: To define the role of CD4 T cells in modulating CDS T cell responses to acute HCV infection. The number, phenotype, and compartmentalization of CD4 cells will be investigated. Measures will include TH1/TH2 cytokine expression, cell surface markers of phenotype and regulation, and the temporal relationship of these markers versus CDS function determined in aim 1. The proposed studies will be synergistic with a well-characterized prospective cohort of active injection drug users (see clinical core) from whom clinical, histologic, and virologic information will also be obtained.